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United States Patent |
5,504,764
|
Pohlmann
,   et al.
|
April 2, 1996
|
Micro-heatpipe cooling of solid-state slab
Abstract
A solid-state laser slab amplifier arrangement for the output of a laser
m at optimal working conditions. Pump energy is applied to a solid state
lasing material so as to irradiate the solid-state lasing material and
effect a laser beam with resultant waste heat output. At least one
micro-heatpipe bundle which includes multiple micro-heatpipes allows waste
heat to be transported away by the working medium through an approximate
center of an individual micro-heatpipe from the solid-state lasing
material. After condensation, liquid working medium is returned
substantially by capillary action through approximate off-center areas of
the individual micro-heatpipe.
Inventors:
|
Pohlmann; Juergen L. W. (Alexandria, VA);
Minch; Richard B. (Woodinville, WA)
|
Assignee:
|
The United States of America as represented by the Secretary of the Army (Washington, DC)
|
Appl. No.:
|
352025 |
Filed:
|
November 30, 1994 |
Current U.S. Class: |
372/34; 372/35; 372/36 |
Intern'l Class: |
H01S 003/00 |
Field of Search: |
372/34,35,36
|
References Cited
U.S. Patent Documents
4791634 | Dec., 1988 | Miyake | 372/34.
|
5253260 | Oct., 1993 | Palombo | 372/34.
|
5394426 | Feb., 1995 | Joslin | 372/36.
|
Primary Examiner: Bovernick; Rodney B.
Assistant Examiner: Wise; Robert E.
Attorney, Agent or Firm: Lee; Milton W., Bashore; Alain L., Holford; John E.
Claims
Having thus described the invention, what we claim as new and desire to
secure by Letters Patent is as follows:
1. A solid-state laser slab amplifier arrangement for the output of a laser
beam at optimal working conditions including:
a solid-state lasing material;
means for applying pump energy to said solid-state lasing material so as to
irradiate said solid-state lasing material, thus effecting from the
irradiated solid-state lasing material a laser beam and waste heat output;
at least one heatpipe bundle which includes multiple heatpipes, said bundle
coupled to said solid-state lasing material, such that in each of the
individual heatpipes there is effected waste heat transported away by the
working medium through an approximate center of an individual heatpipe
from the solid-state lasing material, and after condensation there is
effected liquid working medium return substantially by capillary action
through approximate off-center areas of the individual heatpipe, such that
the heat exchange working medium does not contact said solid-state lasing
material and there is also effected heat removal that is substantially
proportional to demand, and heat removal directly from or approximate to
points of origin, thereby resulting in a predetermined operating
temperature range being achieved.
2. The solid-state laser slab amplifier arrangement of claim 1 wherein the
heatpipe is a micro-heatpipes further charactereized in that each
micro-heatpipe has an approximate diameter on the order of millimeters or
fractions thereof.
3. A technique for optimizing the lasing conditions of a solid-state laser
slab amplifier, including the steps of:
providing a solid-state lasing material;
irradiating said solid-state lasing material for effecting from the
irradiated solid state lasing material a laser beam output with an
undesirable waste heat component;
transporting the waste heat away from the lasing material by a vapor phase
working medium through individual, whereby the vapor is condensed and
returned to the working medium for further cooling of the laser slab such
that no heat exchange working medium contacts said solid-state lasing
material, thereby effecting heat removal that is substantially
proportional to demand, and heat removal directly from or approximate to
points of origin, thereby resulting in a predetermined operating
temperature range being achieved.
4. The technique for optimizing the lasing conditions of a solid-state
laser slab amplifier of claim 3, further including the step that each
heatpipe is fabricated as a micro-heatpipe further charactereized in that
each micro-heatpipe has an approximate diameter on the order of
millimeters or fractions thereof.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optically pumped solid-state
lasers and more specifically, to a device, arrangement, and method for
removing excess heat from a solid-state lasing slab in order to stabilize
the lasing operation.
2. Description of Prior Art
Generally, the conversion of incoherent optical energy into coherent
optical radiation is accompanied by the generation of heat which must be
removed from the device essentially instantaneously. While much effort has
been made to efficiently and effectively remove waste heat from diode
laser arrays, the removal of waste heat from the solid-state lasing
material has been found to be more challenging. Cooling of the lasing
medium avoids or negates the build-up of temperature gradients and thereby
the strain and stress in the lasing host material. Thus, variation of the
refractive index and optical distortion can be largely controlled or
avoided. The immediate result is improved beam quality and/or increased
average power in the laser output. The removal of waste heat and the
concurrent thermal management of the lasing medium also is the most
important issue and the premiere factor limiting scale-up to compact,
high-power solid-state laser devices.
The currently most energy-efficient method to generate high-power, coherent
optical radiation, utilizes the output of a number of compact diode laser
arrays to pump a solid-state medium containing the lasant or lasing
species of choice. While the prior art has reported using cooling systems
and techniques to cool diode laser arrays, none have established a basis
for a specific apparatus that is dedicated to the task of resolving the
particular problem at hand. What is needed in this instance is an
apparatus and technique for the removal of thermal energy from solid-state
lasing materials without direct heat exchange medium contact such that
there is no performance deterioration.
SUMMARY OF THE INVENTION
It is therefore one object of the invention to provide an apparatus and
technique for the removal of thermal energy from solid-state lasing
materials without direct heat exchange medium contact such that there is
no performance deterioration.
There is disclosed a solid-state laser slab amplifier technique and
arrangement for the output of a laser beam at optimal working conditions.
A solid-state lasing material is provided which is irradiated thus
effecting a laser beam output and waste heat. A micro-heatpipe bundle
allows for passively removing waste heat from the irradiated lasing
material such that no heat exchange working medium contacts the
solid-state lasing material, thereby effecting heat removal that is
substantially proportional to demand resulting in a predetermined
operating temperature range being achieved.
The embodiment disclosed is operated in a master oscillator/power amplifier
(MOPA) configuration. A well characterized, coherent seed beam of about 5
Mwatt is radiated into the amplifier slab which receives nominally 1000
Watt optical energy from the diode laser pumping devices. Since the
optical-to-optical conversion efficiency is 40%, the amplified laser beam
has a nominal time-averaged output of 400 Watts in the same pulse format
as provided by the seed beam from the master oscillator. Consequently, the
5 micro-heatpipe bundles utilized, which are thermally bonded by a thin
contact layer to the slab, must remove about 600 Watts of waste heat. This
cooling requirement of 60 Watts per square centimeter exceeds the
performance requirement of forced high-velocity convection of pressurized
gases or liquids, but is accomplished with the micro-heatpipe bundles.
With a working medium of ammonia/water, the amplifier device is thus
biased (or thermostated) for an optimal working condition at 50 degrees
C..degree. operating temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, aspects, and advantages will be better
understood from the following detailed description of a preferred
embodiment of the invention with reference to the drawings, in which:
FIG. 1 is an exploded detail view of the cross section of individual
micro-heatpipes at the evaporator end.
FIG. 2 is a cross section of the micro-heatpipe bundle made up of many
individual micro-heatpipes shown in FIG. 1.
FIG. 3 is a schematic diagram of the present invention as a pumped and
cooled dual configuration in partial exploded view.
FIG. 4 is a schematic diagram of the present invention utilized in an
optically pumped solid-state laser slab system.
FIG. 5 is a cross section of a micro-heatpipe bundle bonded to the side
surface of a solid-state laser slab as utilized in the embodiment of FIG.
4.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Referring now to the drawings, and more particularly to FIG. 1 is an
exploded view detailing an arbitrary cross section of individual
micro-heatpipes 10, where individual micro-heatpipe having a free
cross-sectional area 11. The individual micro-heatpipes are fabricated of
metal or alloy walls 12 each thermally isolated at layer 13. Heat is
transported within each of the micro-heatpipes by a naturally induced
counter-current flow of gaseous and liquified medium. As individual
micro-heatpipes respond to local demand, heat is taken up by evaporation
of the liquid working medium and transported as gas or vapor through the
open center channel 14 to the condenser end where it is dissipated to an
external coolant circulating through an encasement. After condensation of
the vapor at the condenser/radiator end, the working medium returns to the
evaporator area as a liquid by capillary action inside the grooves or
channels 15. The specific working ranges of a micro-heatpipe depend on the
working medium used, for example ammonia (-60.degree. C.--100.degree. C.)
or water (30.degree. C.-100.degree. C.) but are not the same as those for
regular macro-heatpipes as the design parameters used introduce different
flux versus temperature relationships. It is understood that the working
mediums and ranges are illustrative only and are in no way limiting of the
present invention. Micro-heatpipes are available from Xemet, Inc. of
Redond, Wash.
In FIG. 2 there is shown a complete cross section of a micro-heatpipe
bundle 20 utilized in the present invention. Bundle 20 is composed of
individual micro-heatpipes 10 of FIG. 1 densely packed together whereby a
coating or thin layer such as an oxide layer as layer 13 which provides
thermal isolation between the individual micro-heatpipes at the evaporator
end. While the diameter of each individual heatpipe 10 is in the order of
a fraction to a few millimeters, the length and the dimension of the
bundle is determined by the overall system design and is usually 5
centimeter or longer. A rectangular cross-section is shown in FIG. 2 and
is utilized in the present invention because it corresponds best to the
cross-section of the pumping heads and fits the geometry of the glass
slab. However, any other configuration can be produced to conform to the
lasing glass and to achieve best possible heat exchange. Micro-heatpipes
are thermally contacted to the surface of solid-state material and do not
interfere with optical requirements for the laser rod or slab. No
vibration or other mechanical disturbances affecting the output of the
optical beam is caused by the micro-heatpipe.
FIG. 3 is a schematic diagram in partial exploded view of the present
invention as a side-pumped/cooled dual configuration for the removal of
thermal energy from a solid-state lasing medium 30. It is understood that
the invention is not limited to a particular lasing medium but that any
solid-state lasing medium may be used in any practical geometric
configuration. As shown in subassemblies 32 and 33, cooling micro-heatpipe
bundles 36 and pumping heads 31 alternate side by side and in
juxtaposition along the solid-state laser medium 30. It is also understood
that the invention is not limited to a specific pumping head means, but
that any method generating optical radiation may be utilized. Pumping head
means may include flashlamps, arclamps, chemical reactors, or laser diode
arrays. Only part of the energy 34 radiated into lasing medium 30 is
utilized to amplify laser beam 35, the rest generates absorption heat
which must be removed. The evaporator or radiator ends 37 of the
micro-heatpipes are in sufficient thermal contact with 30 through use of a
thin bonding layer and are physically located next to the pumping heads
31. This assures an immediate removal of the absorption heat without
significant lateral heat spread, as each individual micro-heatpipe
responds to the demand it encounters. This response mechanism is
especially effective when oxide coatings thermally isolate the individual
micro-heatpipes against each other for a length that exceeds the heat
diffusion depth in the material they are made of.
For optimal thermal management, pumping areas and heatsinking areas would
be finely interlaced to obtain the most uniform operating conditions
during lasing. Micro-heatpipes are closed systems; by proper selection of
the working fluid, the lasing medium can be stabilized at its optimal
operational condition, independent of the temperature requirement of the
laser diode-arrays or other pumping heads. Configuration of the
Micro-heatpipes can be in such a way that the heat dissipation is
accomplished far away from the laser rod or slab, leaving maximum surface
area and working volume available for the pumping diode-arrays or other
radiation sources. Failure of an individual micro-heatpipe will be
compensated for by its closest neighbors, and does not automatically lead
to shut-down or self-destruction of the laser device. The operation of
micro-heatpipes does not depend on gravity, and thereby imposes few or no
design constraints. This fact makes micro-heatpipes very attractive for
cooling operations in space.
FIG. 4 is a schematic diagram of the present invention as utilized in a
side-pumped solid-state laser slab amplifier. Solid-state laser slab 40 is
a Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG), approximately 10 cm
ling, 4 cm wide, and 1 cm high, with Brewster angle 41 cuts on both faces.
The pumping laser diode arrays 31 have a face area about 2 cm wide and 1
cm high and provide time-averaged optical energy of approximately 100 Watt
per square centimeter at a wavelength of 804 nm. The detailed arrangement
of the diode lasers is shown for one pumping head. The individual, linear
diode bars 45 are mounted on the front end of heatsinks 46 and stacked to
a height of 1 cm. The divergent radiation of all laser bars in that stack
is collected by a cylindrical lens 44 and focused into laser slab 30 that
has an anti-reflection coating 42 on all side surfaces.
The device shown in FIG. 4 is operated in a master oscillator power
amplifier (MOPA) configuration. A well characterized, coherent seed beam
43 of about 5 Mwatt is radiated into the amplifier slab which receives
nominally 1000 Watt optical energy from the diode laser pumping devices.
Since the optical-to-optical conversion efficiency is 40%, the amplified
laser beam 47 has a nominal time-averaged output of 400 Watts in the same
pulse format as provided by the seed beam from the master oscillator.
Consequently, the 5 micro-heatpipe bundles 36, which are thermally bonded
by a thin contact layer 39 to the slab, must remove about 600 Watts of
waste heat. This cooling requirement of 60 Watts per square centimeter
exceeds the performance requirement of forced high-velocity convection of
pressurized gases or liquids, but can be accomplished with
micro-heatpipes. By judicious selection of an appropriate working medium,
which in this embodiment is ammonia/water, the amplifier device is biased
(or thermostated) for optimal working condition at 50 degrees operating
temperature.
FIG. 5 depicts the cross section of a solid-state laser slab 30 with a
micro-heatpipe bundle 20 bonded to its side surface utilized in the
embodiment of FIG. 4. With individual micro-heatpipes 10 responding to
local demand, heat is transported by the gaseous working medium through
the open center channel 14 to the condenser/radiation end area 51 and
dissipated to an external coolant circulating through encasement 52. After
condensation at area 51, the liquid working medium returns by capillary
action in the side channels 15 to the evaporator end area 53. The overall
length of the micro-heatpipe bundle 20 is determined by the engineering
requirements. Heat is removed directly from points of origin or from
surface area close to origin. Heat removal is passive and proportional to
demand; hot spots are negated or avoided. The heat exchange medium
(working fluid) does not get into contact with laser glass and therefore
does not leach out or erode optical material of rod or slab.
While this invention has been described in terms of preferred embodiment
consisting of a pumped solid-state laser slab system, those skilled in the
art will recognize that the invention can be practiced with modification
within the spirit and scope of the appended claims.
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